Double gate strained-semiconductor-on-insulator device structures

ABSTRACT

The benefits of strained semiconductors are combined with silicon-on-insulator approaches to substrate and device fabrication.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application 60/386,968 filed Jun. 7, 2002, U.S. Provisional Application 60/404,058 filed Aug. 15, 2002, and U.S. Provisional Application 60/416,000 filed Oct. 4, 2002; the entire disclosures of these three provisional applications are hereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates to devices and structures comprising strained semiconductor layers and insulator layers.

BACKGROUND

Strained silicon-on-insulator structures for semiconductor devices combine the benefits of two advanced approaches to performance enhancement: silicon-on-insulator (SOI) technology and strained silicon (Si) technology. The strained silicon-on-insulator configuration offers various advantages associated with the insulating substrate, such as reduced parasitic capacitances and improved isolation. Strained Si provides improved carrier mobilities. Devices such as strained Si metal-oxide-semiconductor field-effect transistors (MOSFETs) combine enhanced carrier mobilities with the advantages of insulating substrates.

Strained-silicon-on-insulator substrates are typically fabricated as follows. First, a relaxed silicon-germanium (SiGe) layer is formed on an insulator by one of several techniques such as separation by implantation of oxygen (SIMOX), wafer bonding and etch back; wafer bonding and hydrogen exfoliation layer transfer; or recrystallization of amorphous material. Then, a strained Si layer is epitaxially grown to form a strained-silicon-on-insulator structure, with strained Si disposed over SiGe. The relaxed-SiGe-on-insulator layer serves as the template for inducing strain in the Si layer. This induced strain is typically greater than 10⁻³.

This structure has limitations. It is not conducive to the production of fully-depleted strained-semiconductor-on-insulator devices in which the layer over the insulating material must be thin enough [<300 angstroms (Å)] to allow for full depletion of the layer during device operation. Fully depleted transistors may be the favored version of SOI for MOSFET technologies beyond the 90 nm technology node. The relaxed SiGe layer adds to the total thickness of this layer and thus makes it difficult to achieve the thicknesses required for fully depleted silicon-on-insulator device fabrication. The relaxed SiGe layer is not required if a strained Si layer can be produced directly on the insulating material. Thus, there is a need for a method to produce strained silicon—or other semiconductor—layers directly on insulating substrates.

Double-gate MOSFETs

Double gate MOSFETs have the potential for superior performance in comparison to standard single-gate bulk or single-gate SOI MOSFET devices. This is due to the fact that two gates (one above and one below the channel) allow much greater control of channel charge then a single gate. This configuration has the potential to translate to higher drive current and lower stand-by leakage current.

finFETs

Fin-field-effect transistors (finFETs), like double-gate MOSFETs, typically have two gates (one on either side of the channel, where the channel is here oriented vertically) allowing much greater control of channel charge than in a single gate device. This configuration also has the potential to translate to higher drive current and lower stand-by leakage current. Devices related to the finFET, such as the wrap-around gate FET (gate on both sides of as well as above the channel) allow even more channel charge control and hence even more potential for improved drive current and leakage current performance.

Bipolar-CMOS

The bipolar-CMOS (BiCMOS) process is a combination of both the bipolar transistor and MOSFET/CMOS processes. Individually, the CMOS process allows low power dissipation, high packing density and the ability to integrate complexity with high-speed yields. A major contribution to power dissipation in CMOS circuits originates from driving the load capacitance that is usually the gate of sequentially linked logic cells. The size of these gates may be kept sufficiently small, but when driving higher loads (such as input/output buffers or data buses) the load or capacitance of such devices is substantially larger and therefore requires greater gate width (hence area) of transistor, which inevitably drives down the switching speed of the MOSFET.

The bipolar transistor has significant advantages in terms of the drive current per unit active area and reduced noise signal. Additionally, the switching speed is enhanced due to the effectively exponential output current swing with respect to input signal. This means that the transconductance of a bipolar transistor is significantly higher than that of a MOS transistor when the same current is passed. Higher transconductance enables the charging process to take place approximately ten times more quickly in emitter coupled logic circuits, or high fan out/load capacitance.

Pure bipolar technology has not replaced the high packing density microprocessor CMOS process for a number of reasons, including issues of yield and the increased area required for device isolation. However, integration of bipolar and CMOS may provide the best aspects of the composite devices.

-   -   The advantages of BiCMOS process may be summarized as follows:     -   1. Improved speed performance of highly integrated functionality         of CMOS technology;     -   2. Lower power dissipation than bipolar technology;     -   3. Lower sensitivity to fan out and capacitive load;     -   4. Increased flexibility of input/output interface;     -   5. Reduced clock skew;     -   6. Improved internal gate delay; and     -   7. Reduced need for aggressive scaling because a 1-2 μm BiCMOS         process offers circuit speed equivalent to that of sub-micron         CMOS.

SUMMARY

The present invention includes a strained-semiconductor-on-insulator (SSOI) substrate structure and methods for fabricating the substrate structure. MOSFETs fabricated on this substrate will have the benefits of SOI MOSFETs as well as the benefits of strained Si mobility enhancement. For example, the formation of BiCMOS structures on SSOI substrates provides the combined benefits of BiCMOS design platforms and enhanced carrier mobilities. SSOI substrates also enable enhanced carrier mobilities, process simplicity, and better device isolation for double-gate MOSFETs and finFETs.

By eliminating the SiGe relaxed layer traditionally found beneath the strained Si layer, the use of SSOI technology is simplified. For example, issues such as the diffusion of Ge into the strained Si layer during high temperature processes are avoided.

This approach enables the fabrication of well-controlled, epitaxially-defined, thin strained semiconductor layers directly on an insulator layer. Tensile strain levels of ˜10⁻³ or greater are possible in these structures, and are not diminished after thermal anneal cycles. In some embodiments, the strain-inducing relaxed layer is not present in the final structure, eliminating some of the key problems inherent to current strained Si-on-insulator solutions. This fabrication process is suitable for the production of enhanced-mobility substrates applicable to partially or fully depleted SSOI technology.

In an aspect, the invention features a structure including a substrate having a dielectric layer disposed thereon and a fin-field-effect transistor disposed over the substrate. The fin-field-effect-transistor includes a source region and a drain region disposed in contact with the dielectric layer, the source and the drain regions includinga strained semiconductor material. The fin-field-effect-transistor also includes at least one fin extending between the source and the drain regions, the fin including a strained semiconductor material. A gate is disposed above the strained semiconductor layer, extending over at least one fin and between the source and the drain regions. A gate dielectric layer is disposed between the gate and the fin.

One or more of the following features may be included. The fin may include at least one of a group II, a group III, a group IV, a group V, of a group VI element. The strained semiconductor layer may be tensilely strained and may include, e.g., tensilely strained silicon. The strained semiconductor layer may be compressively strained and may include, e.g., compressively strained germanium.

In another aspect, the invention features a method for forming a structure, the method including providing a substrate having a dielectric layer disposed thereon, and a first strained semiconductor layer disposed in contact with the dielectric layer. A fin-field-effect transistor is formed on the substrate by patterning the first strained semiconductor layer to define a source region, a drain region, and at least one fin disposed between the source and the drain regions. A dielectric layer is formed, at least a portion of the dielectric layer being disposed over the fin, and a gate is formed over the dielectric layer portion disposed over the fin.

One or more of the following features may be included. The first strained semiconductor layer may include at least one of a group II, a group III, a group IV, a group V, or a group VI element. The strained semiconductor layer may be tensilely strained and may include, e.g., tensilely strained silicon. The strained semiconductor layer may be compressively strained and may include, e.g., compressively strained germanium.

In another aspect, the invention features a structure including a dielectric layer disposed over a substrate; and a transistor formed over the dielectric layer. The transistor includes a first gate electrode in contact with the dielectric layer, a strained semiconductor layer disposed over the first gate electrode; and a second gate electrode disposed over the strained semiconductor layer.

One or more of the following features may be included. The strained semiconductor layer may include at least one of a group II, a group III, a group IV, a group V, and a group VI elements.

The strained semiconductor layer may be tensilely strained and may include, e.g., tensilely strained silicon. The strained semiconductor layer may be compressively strained and may include, e.g., compressively strained germanium. The strained semiconductor layer may have a strain level greater than 10⁻³.

A first gate insulator layer may be disposed between the first gate electrode and the strained semiconductor layer. A second gate insulator layer may be disposed between the strained semiconductor layer and the second gate electrode. The strained semiconductor layer may include a source. The strained semiconductor layer may include a drain. A sidewall spacer may be disposed proximate the second gate electrode. The sidewall spacer may include a dielectric or a conductive material.

In another aspect, the invention features a method for forming a structure, the method including forming a substrate having a first gate electrode layer disposed over a substrate insulator layer, a first gate insulator layer disposed over the first gate electrode layer, and a strained semiconductor layer disposed over the first gate insulator layer. A second gate insulator layer is formed over the strained semiconductor layer, and a second gate electrode layer is formed over the second gate insulator layer. A second gate electrode is defined by removing a portion of the second gate insulator layer. A dielectric sidewall spacer is formed proximate the second gate electrode. A portion of the strained semiconductor layer, a portion of the first gate insulator layer, and a portion of the first gate electrode layer are removed to define a vertical structure disposed over the substrate insulator layer, the vertical structure including a strained layer region, a first gate insulator region, and-a first gate electrode layer region disposed under the second gate electrode. A first gate electrode is defined by laterally shrinking the first gate electrode layer region.

One or more of the following features may be included. The strained semiconductor layer may be tensilely strained and may include, e.g., tensilely strained silicon. The strained semiconductor layer may be compressively strained and may include compressively strained germanium. A conductive sidewall spacer may be formed proximate the dielectric sidewall spacer. A source and/or a drain may be defined in the strained semiconductor layer.

In another aspect, the invention features a structure including a strained semiconductor layer disposed over a dielectric layer and a bipolar transistor. The bipolar transistor includes a collector disposed in a portion of the strained semiconductor layer, a base disposed over the collector, and an emitter disposed over the base.

One or more of the following features may be included. The strained layer may be tensilely strained and may include, e.g., tensilely strained silicon. The strained layer may be compressively strained.

In another aspect, the invention features a relaxed substrate including a bulk material, a strained layer disposed in contact with the relaxed substrate; and a bipolar transistor. The bipolar transistor includes a collector disposed in a portion of the strained layer, a base disposed over the collector, and an emitter disposed over the base. The strain of the strained layer is not induced by the underlying substrate.

One or more of the following features may be included. The strained layer may be tensilely strained and may include, e.g., tensilely strained silicon. The strained layer may be compressively strained.

In another aspect, the invention features a structure including a relaxed substrate including a bulk material, a strained layer disposed in contact with the relaxed substrate; and a bipolar transistor including. The bipolar transistor includes a collector disposed in a portion of the strained layer, a base disposed over the collector, and an emitter disposed over the base. The strain of the strained layer is independent of a lattice mismatch between the strained layer and the relaxed substrate.

One or more of the following features may be included. The strained layer may be tensilely strained and may include, e.g., tensilely strained silicon. The strained layer may be compressively strained.

In another aspect, the invention includes a method for forming a structure, the method including providing a substrate having a strained semiconductor layer disposed over a dielectric layer, defining a collector in a portion of the strained semiconductor layer; forming a base over the collector; and forming an emitter over the base.

One or more of the following features may be included. The strained layer may be tensilely strained and may include, e.g., tensilely strained silicon. The strained layer may be compressively strained.

In another aspect, the invention includes method for forming a structure, the method including providing a first substrate having a strained layer disposed thereon, the strained layer including a first semiconductor material. The strained layer is bonded to a second substrate, the second substrate including a bulk material. The first substrate is removed from the strained layer, with the strained layer remaining bonded to the bulk semiconductor material. A collector is defined in a portion of the strained layer. A base is formed over the collector; and an emitter is formed over the base. The strain of the strained layer is not induced by the second substrate and the strain is independent of lattice mismatch between the strained layer and the second substrate.

One or more of the following features may be included. The strained layer may be tensilely strained and may include, e.g., tensilely strained silicon. The strained layer may be compressively strained.

In another aspect, the invention features a method for forming a structure, the method including providing a relaxed substrate comprising a bulk material and a strained layer disposed in contact with the relaxed substrate, the strain of the strained layer not being induced by the underlying substrate and the strain being independent of a lattice mismatch between the strained layer and the relaxed substrate. A collector is defined in a portion of the strained layer. A base is formed over the collector, and an emitter is formed over the base.

One or more of the following features may be included. The strained layer may be tensilely strained and may include, e.g., tensilely strained silicon. The strained layer may be compressively strained.

In another aspect, the invention features a method for forming a structure, the method includes providing a substrate having a strained semiconductor layer disposed over a substrate dielectric layer and forming a transistor in the strained layer. Forming the transistor includes forming a gate dielectric layer above a portion of the strained semiconductor layer, forming a gate contact above the gate dielectric layer, and forming a source region and a drain region in a portion of the strained semiconductor layer, proximate the gate dielectric layer. A portion of the strained layer and the substrate dielectric layer are removed to expose a portion of the substrate. A collector is defined in the exposed portion of the substrate. A base is formed over the collector; and an emitter is formed over the base.

One or more of the following features may be included. The strained layer may be tensilely strained and may include, e.g., tensilely strained silicon. The strained layer may be compressively strained.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A-6 are schematic cross-sectional views of substrates illustrating a method for fabricating an SSOI substrate;

FIG. 7 is a schematic cross-sectional view illustrating an alternative method for fabricating the SSOI substrate illustrated in FIG. 6;

FIG. 8 is a schematic cross-sectional view of a transistor formed on the SSOI substrate illustrated in FIG. 6;

FIGS. 9-10 are schematic cross-sectional views of substrate(s) illustrating a method for fabricating an alternative SSOI substrate;

FIG. 11 is a schematic cross-sectional view of a substrate having several layers formed thereon;

FIGS. 12-13 are schematic cross-sectional views of substrates illustrating a method for fabricating an alternative strained semiconductor substrate;

FIG. 14 is a schematic cross-sectional view of the SSOI substrate illustrated in FIG. 6 after additional processing;

FIGS. 15-21B are cross-sectional and top views of substrates illustrating a method for fabricating a fin-field-effect transistor (finFET) on an SSOI substrate;

FIGS. 22-35 are cross-sectional views of substrates illustrating a method for fabricating a dual-gate transistor on an SSOI substrate;

FIGS. 36-39 are cross-sectional views of substrates illustrating a method for fabricating a bipolar transistor on an SSOI substrate; and

FIGS. 40A-41D are schematic cross-sectional views of substrates illustrating alternative methods for fabricating an SSOI substrate.

Like-referenced features represent common features in corresponding drawings.

DETAILED DESCRIPTION

An SSOI structure may be formed by wafer bonding followed by cleaving. FIGS 1A-2B illustrate formation of a suitable strained layer on a wafer for bonding, as further described below.

Referring to FIG. 1A, an epitaxial wafer 8 has a plurality of layers 10 disposed over a substrate 12. Substrate 12 may be formed of a semiconductor, such as Si, Ge, or SiGe. The plurality of layers 10 includes a graded buffer layer 14, which may be formed of Si_(1−y)Ge_(y), with a maximum Ge content of, e.g., 10-80% (i.e., y=0.1-0.8) and a thickness T₁ of, for example, 1-8 micrometers (μm).

A relaxed layer 16 is disposed over graded buffer layer 14. Relaxed layer 16 may be formed of uniform Si_(1−x)Ge_(x), having a Ge content of, for example, 10-80% (i.e., x=0.1-0.8), and a thickness T₂ of, for example, 0.2-2 μm. In some embodiments, Si_(1−x)Ge_(x) may include Si_(0.70)Ge_(0.30) and T₂ may be approximately 1.5 μm. Relaxed layer 16 may be fully relaxed, as determined by triple axis X-ray diffraction, and may have a threading dislocation density of <1×10⁶ dislocations/cm², as determined by etch pit density (EPD) analysis. Because threading dislocations are linear defects disposed within a volume of crystalline material, threading dislocation density may be measured as either the number of dislocations intersecting a unit area within a unit volume or the line length of dislocation per unit volume. Threading dislocation density, therefore, may be expressed in either units of dislocations/cm² or cm/cm³. Relaxed layer 16 may have a surface particle density of, e.g., less than about 0.3 particles/cm². Further, relaxed layer 16 produced in accordance with the present invention may have a localized light-scattering defect level of less than about 0.3 defects/cm² for particle defects having a size (diameter) greater than 0.13 microns, a defect level of about 0.2 defects/cm² for particle defects having a size greater than 0.16 microns, a defect level of about 0.1 defects/cm² for particle defects having a size greater than 0.2 microns, and a defect level of about 0.03 defects/cm² for defects having a size greater than 1 micron. Process optimization may enable reduction of the localized light-scattering defect levels to about 0.09 defects/cm² for particle defects having a size greater than 0.09 microns and to 0.05 defects/cm² for particle defects having a size greater than 0.12 microns.

Substrate 12, graded layer 14, and relaxed layer 16 may be formed from various materials systems, including various combinations of group II, group III, group IV, group V, and group VI elements. For example, each of substrate 12, graded layer 14, and relaxed layer 16 may include a III-V compound. Substrate 12 may include gallium arsenide (GaAs), graded layer 14 and relaxed layer 16 may include indium gallium arsenide (InGaAs) or aluminum gallium arsenide (AlGaAs). These examples are merely illustrative, and many other material systems are suitable.

A strained semiconductor layer 18 is disposed over relaxed layer 16. Strained layer 18 may include a semiconductor such as at least one of a group II, a group III, a group IV, a group V, and a group VI element. Strained semiconductor layer 18 may include, for example, Si, Ge, SiGe, GaAs, indium phosphide (InP), and/or zinc selenide (ZnSe). In some embodiments, strained semiconductor layer 18 may include approximately 100% Ge, and may be compressively strained. Strained semiconductor layer 18 comprising 100% Ge may be formed over, e.g., relaxed layer 16 containing uniform Si_(1−x)Ge_(x) having a Ge content of, for example, 50-80% (i.e., x=0.5-0.8), preferably 70% (x=0.7). Strained layer 18 has a thickness T₃ of, for example, 50-1000 Å. In an embodiment, T₃ may be approximately 200-500 Å.

Strained layer 18 may be formed by epitaxy, such as by atmospheric-pressure CVD (APCVD), low- (or reduced-) pressure CVD (LPCVD), ultra-high-vacuum CVD (UHVCVD), by molecular beam epitaxy (MBE), or by atomic layer deposition (ALD). Strained layer 18 containing Si may be formed by CVD with precursors such as silane, disilane, or trisilane. Strained layer 18 containing Ge may be formed by CVD with precursors such as germane or digermane. The epitaxial growth system may be a single-wafer or multiple-wafer batch reactor. The growth system may also utilize a low-energy plasma to enhance layer growth kinetics. Strained layer 18 may be formed at a relatively low temperature, e.g., less than 700° C., to facilitate the definition of an abrupt interface 17 between strained layer 18 and relaxed layer 16. This abrupt interface 17 may enhance the subsequent separation of strained layer 18 from relaxed layer 16, as discussed below with reference to FIGS. 4 and 5. Abrupt interface 17 is characterized by the transition of Si or Ge content (in this example) proceeding in at least 1 decade (order of magnitude in atomic concentration) per nanometer of depth into the sample. In an embodiment, this abruptness may be better than 2 decades per nanometer.

In an embodiment in which strained layer 18 contains substantially 100% Si, strained layer 18 may be formed in a dedicated chamber of a deposition tool that is not exposed to Ge source gases, thereby avoiding cross-contamination and improving the quality of the interface between strained layer 18 and relaxed layer 16. Furthermore, strained layer 18 may be formed from an isotopically pure silicon precursor(s). Isotopically pure Si has better thermal conductivity than conventional Si. Higher thermal conductivity may help dissipate heat from devices subsequently formed on strained layer 18, thereby maintaining the enhanced carrier mobilities provided by strained layer 18.

After formation, strained layer 18 has an initial misfit dislocation density, of, for example, 0-10⁵ cm/cm². In an embodiment, strained layer 18 has an initial misfit dislocation density of approximately 0 cm/cm². Because misfit dislocations are linear defects generally lying within a plane between two crystals within an area, they may be measured in terms of total line length per unit area. Misfit dislocation density, therefore, may be expressed in units of dislocations/cm or cm/cm². In one embodiment, strained layer 18 is tensilely strained, e.g., Si formed over SiGe. In another embodiment, strained layer 18 is compressively strained, e.g., Ge formed over SiGe.

Strained layer 18 may have a surface particle density of, e.g., less than about 0.3 particles/cm². As used herein, “surface particle density” includes not only surface particles but also light-scattering defects, and crystal-originated pits (COPs), and other defects incorporated into strained layer 18. Further, strained layer 18 produced in accordance with the present invention may have a localized light-scattering defect level of less than about 0.3 defects/cm² for particle defects having a size (diameter) greater than 0.13 microns, a defect level of about 0.2 defects/cm² for particle defects having a size greater than 0.16 microns, a defect level of about 0.1 defects/cm² for particle defects having a size greater than 0.2 microns, and a defect level of about 0.03 defects/cm² for defects having a size greater than 1 micron. Process optimization may enable reduction of the localized light-scattering defect levels to about 0.09 defects/cm² for particle defects having a size greater than 0.09 microns and to 0.05 defects/cm² for particle defects having a size greater than 0.12 microns. These surface particles may be incorporated in strained layer 18 during the formation of strained layer 18, or they may result from the propagation of surface defects from an underlying layer, such as relaxed layer 16.

In alternative embodiments, graded layer 14 may be absent from the structure. Relaxed layer 16 may be formed in various ways, and the invention is not limited to embodiments having graded layer 14. In other embodiments, strained layer 18 may be formed directly on substrate 12. In this case, the strain in layer 18 may be induced by lattice mismatch between layer 18 and substrate 12, induced mechanically, e.g., by the deposition of overlayers, such as Si₃N₄, or induced by thermal mismatch between layer 18 and a subsequently grown layer, such as a SiGe layer. In some embodiments, a uniform semiconductor layer (not shown), having a thickness of approximately 0.5 μm and comprising the same semiconductor material as substrate 12, is disposed between graded buffer layer 14 and substrate 12. This uniform semiconductor layer may be grown to improve the material quality of layers subsequently grown on substrate 12, such as graded buffer layer 14, by providing a clean, contaminant-free surface for epitaxial growth. In certain embodiments, relaxed layer 16 may be planarized prior to growth of strained layer 18 to eliminate the crosshatched surface roughness induced by graded buffer layer 14. (See, e.g., M. T. Currie, et al., Appl. Phys. Lett., 72 (14) p. 1718 (1998), incorporated herein by reference.) The planarization may be performed by a method such as chemical mechanical polishing (CMP), and may improve the quality of a subsequent bonding process (see below) because it minimizes the wafer surface roughness and increases wafer flatness, thus providing a greater surface area for bonding.

Referring to FIG. 1B, after planarization of relaxed layer 16, a relaxed semiconductor regrowth layer 19 including a semiconductor such as SiGe may be grown on relaxed layer 16, thus improving the quality of subsequent strained layer 18 growth by ensuring a clean surface for the growth of strained layer 18. Growing on this clean surface may be preferable to growing strained material, e.g., silicon, on a surface that is possibly contaminated by oxygen and carbon from the planarization process. The conditions for epitaxy of the relaxed semiconductor regrowth layer 19 on the planarized relaxed layer 16 should be chosen such that surface roughness of the resulting structure, including layers formed over regrowth layer 19, is minimized to ensure a surface suitable for subsequent high quality bonding. High quality bonding may be defined as the existence of a bond between two wafers that is substantially free of bubbles or voids at the interface. Measures that may help ensure a smooth surface for strained layer 18 growth, thereby facilitating bonding, include substantially matching a lattice of the semiconductor regrowth layer 19 to that of the underlying relaxed layer 16, by keeping the regrowth thickness below approximately 1 μm, and/or by keeping the growth temperature below approximately 850° C. for at least a portion of the semiconductor layer 19 growth. It may also be advantageous for relaxed layer 16 to be substantially free of particles or areas with high threading dislocation densities (i.e., threading dislocation pile-ups) which could induce non-planarity in the regrowth and decrease the quality of the subsequent bond.

Referring to FIG. 2A, in an embodiment, hydrogen ions are implanted into relaxed layer 16 to define a cleave plane 20. This implantation is similar to the SMARTCUT process that has been demonstrated in silicon by, e.g., SOITEC, based in Grenoble, France. Implantation parameters may include implantation of hydrogen (H₂ ⁺) to a dose of 2.5-5×10¹⁶ions/cm² at an energy of, e.g., 50-100 keV. For example, H₂ ⁺ may be implanted at an energy of 75 keV and a dose of 4×10¹⁶ ions/cm² through strained layer 18 into relaxed layer 16. In alternative embodiments, it may be favorable to implant at energies less than 50 keV to decrease the depth of cleave plane 20 and decrease the amount of material subsequently removed during the cleaving process (see discussion below with reference to FIG. 4). In an alternative embodiment, other implanted species may be used, such as H⁺ or He⁺, with the dose and energy being adjusted accordingly. The implantation may also be performed prior to the formation of strained layer 18. Then, the subsequent growth of strained layer 18 is preferably performed at a temperature low enough to prevent premature cleaving along cleave plane 20, i.e., prior to the wafer bonding process. This cleaving temperature is a complex function of the implanted species, implanted dose, and implanted material. Typically, premature cleaving may be avoided by maintaining a growth temperature below approximately 500° C.

In some embodiments, such as when strained layer 18 comprises nearly 100% Ge, a thin layer 21 of another material, such as Si, may be formed over strained layer 18 prior to bonding (see discussion with respect to FIG. 3). This thin layer 21 may be formed to enhance subsequent bonding of strained layer 18 to an insulator, such as an oxide. Thin layer 21 may have a thickness T₂₁ of, for example, 0.5-5 nm.

In some embodiments, strained layer 18 may be planarized by, e.g., CMP, to improve the quality of the subsequent bond. Strained layer 18 may have a low surface roughness, e.g., less than 0.5 nm root mean square (RMS). Referring to FIG. 2B, in some embodiments, a dielectric layer 22 may be formed over strained layer 18 prior to ion implantation into relaxed layer 16 to improve the quality of the subsequent bond. Dielectric layer 22 may be, e.g., silicon dioxide (SiO₂) deposited by, for example, LPCVD or by high density plasma (HDP). An LPCVD deposited SiO₂ layer may be subjected to a densification step at elevated temperature. Suitable conditions for this densification step may be, for example, a 10 minute anneal at 800° C. in a nitrogen ambient. Alternatively, dielectric layer 22 may include low-temperature oxide (LTO), which may be subsequently densified at elevated temperature in nitrogen or oxygen ambients. Suitable conditions for this densification step can be a 10 minute anneal at 800° C. in an oxygen ambient. Dielectric layer 22 may be planarized by, e.g., CMP to improve the quality of the subsequent bond. In an alternative embodiment, it may be advantageous for dielectric layer 22 to be formed from thermally grown SiO₂ in order to provide a high quality semiconductor/dielectric interface in the final structure. In an embodiment, strained layer 18 comprises approximately 100% Ge and dielectric layer 22 comprises, for example, germanium dioxide (GeO₂); germanium oxynitride (GeON); a high-k insulator having a higher dielectric constant than that of Si such as hafnium oxide (HfO₂) or hafinium silicate (HfSiON, HfSiO₄); or a multilayer structure including GeO₂ and SiO₂. Ge has an oxidation behavior different from that of Si, and the deposition methods may be altered accordingly.

Referring to FIG. 3, epitaxial wafer 8 is bonded to a handle wafer 50. Either handle wafer 50, epitaxial wafer 8, or both have a top dielectric layer (see, e.g., dielectric layer 22 in FIG. 2B) to facilitate the bonding process and to serve as an insulator layer in the final substrate structure. Handle wafer 50 may have a dielectric layer 52 disposed over a semiconductor substrate 54. Dielectric layer 52 may include, for example, SiO₂. In an embodiment, dielectric layer 52 includes a material having a melting point (T_(m)) higher than a T_(m) of pure SiO₂, i.e., higher than 1100° C. Examples of such materials are silicon nitride (Si₃N₄), aluminum oxide, magnesium oxide, etc. Using dielectric layer 52 with a high T_(m) helps prevents possible relaxation of the transferred strained semiconductor layer 18 that may occur during subsequent processing, due to softening of the underlying dielectric layer 52 at temperatures typically used during device fabrication (approximately 1000-1200° C.). In other embodiments, handle wafer 50 may include a combination of a bulk semiconductor material and a dielectric layer, such as a silicon on insulator substrate. Semiconductor substrate 54 includes a semiconductor material such as, for example, Si, Ge, or SiGe.

Handle wafer 50 and epitaxial wafer 8 are cleaned by a wet chemical cleaning procedure to facilitate bonding, such as by a hydrophilic surface preparation process to assist the bonding of a semiconductor material, e.g., strained layer 18, to a dielectric material, e.g., dielectric layer 52. For example, a suitable prebonding surface preparation cleaning procedure could include a modified megasonic RCA SC1 clean containing ammonium hydroxide, hydrogen peroxide, and water (NH₄OH:H₂O₂:H₂O) at a ratio of 1:4:20 at 60° C. for 10 minutes, followed by a deionized (DI) water rinse and spin dry. The wafer bonding energy should be strong enough to sustain the subsequent layer transfer (see FIG. 4). In some embodiments, top surfaces 60, 62 of handle wafer 50 and epitaxial wafer 8, including a top surface 63 of strained semiconductor layer 18, may be subjected to a plasma activation, either before, after, or instead of a wet clean, to increase the bond strength. The plasma environment may include at least one of the following species: oxygen, ammonia, argon, nitrogen, diborane, and phosphine. After an appropriate cleaning step, handle wafer 50 and epitaxial wafer 8 are bonded together by bringing top surfaces 60, 62 in contact with each other at room temperature. The bond strength may be greater than 1000 mJ/m², achieved at a low temperature, such as less than 600° C.

Referring to FIG. 4 as well as to FIG. 3, a split is induced at cleave plane 20 by annealing handle wafer 50 and epitaxial wafer 8 after they are bonded together. This split may be induced by an anneal at 300-700° C., e.g., 550° C., inducing hydrogen exfoliation layer transfer (i.e., along cleave plane 20) and resulting in the formation of two separate wafers 70, 72. One of these wafers (70) has a first portion 80 of relaxed layer 16 (see FIG. 1A) disposed over strained layer 18. Strained layer 18 is in contact with dielectric layer 52 on semiconductor substrate 54. The other of these wafers (72) includes substrate 12, graded layer 14, and a remaining portion 82 of relaxed layer 16. In some embodiments, wafer splitting may be induced by mechanical force in addition to or instead of annealing. If necessary, wafer 70 with strained layer 18 may be annealed further at 600-900° C., e.g., at a temperature greater than 800° C., to strengthen the bond between the strained layer 18 and dielectric layer 52. In some embodiments, this anneal is limited to an upper temperature of about 900° C. to avoid the destruction of a strained Si/relaxed SiGe heterojunction by diffusion. Wafer 72 may be planarized, and used as starting substrate 8 for growth of another strained layer 18. In this manner, wafer 72 may be “recycled” and the process illustrated in FIGS. 1A-5 may be repeated. An alternative “recyling” method may include providing relaxed layer 16 that is several microns thick and repeating the process illustrated in FIGS. 1A-5, starting with the formation of strained layer 18. Because the formation of this thick relaxed layer 16 may lead to bowing of substrate 12, a layer including, e.g., oxide or nitride, may be formed on the backside of substrate 12 to counteract the bowing. Alternatively substrate 12 may be pre-bowed when cut and polished, in anticipation of the bow being removed by the formation of thick relaxed layer 16.

Referring to FIG. 4 as well as to FIG. 5, relaxed layer portion 80 is removed from strained layer 18. In an embodiment, removal of relaxed layer portion 80, containing, e.g., SiGe, includes oxidizing the relaxed layer portion 80 by wet (steam) oxidation. For example, at temperatures below approximately 800° C., such as temperatures between 600-750° C., wet oxidation will oxidize SiGe much more rapidly then Si, such that the oxidation front will effectively stop when it reaches the strained layer 18, in embodiments in which strained layer 18 includes Si. The difference between wet oxidation rates of SiGe and Si may be even greater at lower temperatures, such as approximately 400° C.-600° C. Good oxidation selectivity is provided by this difference in oxidation rates, i.e., SiGe may be efficiently removed at low temperatures with oxidation stopping when strained layer 18 is reached. This wet oxidation results in the transformation of SiGe to a thermal insulator 90, e.g., Si_(x)Ge_(y)O_(x). The thermal insulator 90 resulting from this oxidation is removed in a selective wet or dry etch, e.g., wet hydrofluoric acid. In some embodiments, it may be more economical to oxidize and strip several times, instead of just once.

In certain embodiments, wet oxidation may not completely remove the relaxed layer portion 80. Here, a localized rejection of Ge may occur during oxidation, resulting in the presence of a residual Ge-rich SiGe region at the oxidation front, on the order of, for example, several nanometers in lateral extent. A surface clean may be performed to remove this residual Ge. For example, the residual Ge may be removed by a dry oxidation at, e.g., 600° C., after the wet oxidation and strip described above. Another wet clean may be performed in conjunction with—or instead of—the dry oxidation. Examples of possible wet etches for removing residual Ge include a Piranha etch, i.e., a wet etch that is a mixture of sulfuric acid and hydrogen peroxide (H₂SO₄:H₂O₂) at a ratio of, for example, 3:1. An HF dip may be performed after the Piranha etch. Alternatively, an RCA SC1 clean may be used to remove the residual Ge. The process of Piranha or RCA SC1 etching and HF removal of resulting oxide may be repeated more than once. In an embodiment, relaxed layer portion including, e.g., SiGe, is removed by etching and annealing under a hydrochloric acid (HCl) ambient.

In the case of a strained Si layer, the surface Ge concentration of the final strained Si surface is preferably less than about 1×10¹² atoms/cm² when measured by a technique such as total reflection x-ray fluorescence (TXRF) or the combination of vapor phase decomposition (VPD) with a spectroscopy technique such as graphite furnace atomic absorption spectroscopy (GFAAS) or inductively-coupled plasma mass spectroscopy (ICP-MS). In some embodiments, after cleaving, a planarization step or a wet oxidation step may be performed to remove a portion of the damaged relaxed layer portion 80 as well as to increase the smoothness of its surface. A smoother surface may improve the uniformity of subsequent complete removal of a remainder of relaxed layer portion 80 by, e.g., wet chemical etching. After removal of relaxed layer portion 80, strained layer 18 may be planarized. Planarization of strained layer 18 may be performed by, e.g., CMP; an anneal at a temperature greater than, for example, 800° C., in a hydrogen (H₂) or hydrochloric acid (HCl) containing ambient; or cluster ion beam smoothing.

Referring to FIG. 6, a SSOI substrate 100 has strained layer 18 disposed over an insulator, such as dielectric layer 52 formed on semiconductor substrate 54. Strained layer 18 has a thickness T₄ selected from a range of, for example, 50-1000 Å, with a thickness uniformity of better than approximately ±5% and a surface roughness of less than approximately 20 Å. Dielectric layer 52 has a thickness T₅₂ selected from a range of, for example, 500-3000 Å. In an embodiment, strained layer 18 includes approximately 100% Si or 100% Ge having one or more of the following material characteristics: misfit dislocation density of, e.g., 0-10⁵ cm/cm²; a threading dislocation density of about 10¹-10⁷ dislocations/cm²; a surface roughness of approximately 0.01-1 nm RMS; and a thickness uniformity across SSOI substrate 100 of better than approximately ±10% of a mean desired thickness; and a thickness T₄ of less than approximately 200 Å. In an embodiment, SSOI substrate 100 has a thickness uniformity of better than approximately ±5% of a mean desired thickness.

In an embodiment, dielectric layer 52 has a T_(m) greater than that of SiO₂. During subsequent processing, e.g., MOSFET formation, SSOI substrate 100 may be subjected to high temperatures, i.e., up to 1100° C. High temperatures may result in the relaxation of strained layer 18 at an interface between strained layer 18 and dielectric layer 52. The use of dielectric layer with a T_(m) greater than 1700° C. may help keep strained layer 18 from relaxing at the interface between strained layer 18 and dielectric layer 52 when SSOI substrate is subjected to high temperatures.

In an embodiment, the misfit dislocation density of strained layer 18 may be lower than its initial dislocation density. The initial dislocation density may be lowered by, for example, performing an etch of a top surface 92 of strained layer 18. This etch may be a wet etch, such as a standard microelectronics clean step such as an RCA SC1, i.e., hydrogen peroxide, ammonium hydroxide, and water (H₂O₂+NH₄OH+H₂O), which at, e.g., 80° C. may remove silicon.

The presence of surface particles on strained layer 18, as described above with reference to FIG. 1A, may result in the formation of bonding voids at an interface 102 between strained layer 18 and dielectric layer 52. These bonding voids may have a density equivalent to the density of surface particles formed on strained layer 18, e.g., less than about 0.3 voids/cm².

In some embodiments, strained semiconductor layer 18 includes Si and is substantially free of Ge; further, any other layer disposed in contact with strained semiconductor layer 18 prior to device processing, e.g., dielectric layer 52, is also substantially free of Ge.

Referring to FIG. 7, in an alternative embodiment, relaxed layer portion 80 may be removed by a selective wet etch that stops at the strained layer 18 to obtain SSOI substrate 100 (see FIG. 6). In embodiments in which relaxed layer portion 80 contains SiGe, a suitable selective SiGe wet etch may be a solution containing nitric acid (HNO₃) and dilute HF at a ratio of 3:1 or a solution containing H₂O₂, HF, and acetic acid (CH₃COOH) at a ratio of 2:1:3. Alternatively, relaxed layer portion 80 may be removed by a dry etch that stops at strained layer 18. In some embodiments, relaxed layer portion 80 may be removed completely or in part by a chemical-mechanical polishing step or by mechanical grinding.

Strained semiconductor-on-insulator substrate 100 may be further processed by CMOS SOI MOSFET fabrication methods. For example, referring to FIG. 8A, a transistor 200 may be formed on SSOI substrate 100. Forming transistor 200 includes forming a gate dielectric layer 210 above strained layer 18 by, for example, growing an SiO₂ layer by thermal oxidation. Alternatively, gate dielectric layer 210 may include a high-k material with a dielectric constant higher than that of SiO₂, such as HfO₂, HfSiON, or HfSiO₄. In some embodiments, gate dielectric layer 210 may be a stacked structure, e.g., a thin SiO₂ layer capped with a high-k material. A gate 212 is formed over gate dielectric layer 210. Gate 212 may be formed of a conductive material, such as doped semiconductor, e.g., polycrystalline Si or polycrystalline SiGe; a metal, e.g., titanium (Ti), tungsten (W), molybdenum (Mo), tantalum (Ta), nickel (Ni), or iridium (Ir); or metal compounds, e.g., titanium nitride (TiN), titanium silicon nitride (TiSiN), tungsten nitride (WN), tantalum nitride (TaN), tantalum silicide (TaSi), nickel silicide (NiSi), or iridium oxide (IrO₂), that provide an appropriate workfunction. A source region 214 and a drain region 216 are formed in a portion 218 of strained semiconductor layer 18, proximate gate dielectric layer 210. Source and drain regions 214, 216 may be formed by, e.g., ion implantation of either n-type or p-type dopants.

In some embodiments, strained semiconductor layer 18 may be compressively strained when, for example, layer 18 includes strained Ge. Compressively strained layers may be prone to undulation when subjected to large temperature changes. The risk of such undulation may be reduced by reducing the thermal budget of a process for fabricating devices, such as transistor 200. The thermal budget may reduced by, for example, using atomic layer deposition (ALD) to deposit gate dielectric layer 210. Furthermore, a maximum temperature for forming gate 212 may be limited to, e.g., 600° C. by, for example, the use of materials comprising metal or metal compounds, rather than polysilicon or other gate materials that may require higher formation and/or dopant activation temperatures.

Referring to FIG. 8B, a transistor 250 formed on SSOI substrate 100 may have an elevated source region and an elevated drain region proximate a first and a second sidewall spacer 252, 254. These elevated regions may be formed as follows. A semiconductor layer 256 a-256 c is formed selectively on exposed silicon surfaces, i.e., on top surface 258 of a gate 259 containing silicon, a top surface 260 of a source 262 defined in strained layer 18, and top surface 264 of a drain 266 defined in strained layer 18. In an embodiment, semiconductor layer 256 a-256 c is an epitaxial layer, such as epitaxial silicon, epitaxial germanium, or epitaxial silicon-germanium. No semiconductor layer is formed on non-silicon features, such as sidewall spacers 252, 254 and dielectric isolation regions 268, 270. Semiconductor layer 256 a-256 c has a thickness T₂₅₆ of, for example, approximately 100-500 Å.

Semiconductor layer 256 a-256 c has a low resistivity of, e.g., 0.001 ohm-cm, that facilitates the formation of low-resistance contacts. To achieve this low resistivity, semiconductor layer 256 a-256 c is, for example, epitaxial silicon doped with, for example, arsenic to a concentration of 1×10²⁰ atoms/cm³. Semiconductor layer 256 a-256 c may be doped in situ, during deposition. In alternative embodiments, semiconductor layer 256 a-256 c may be doped after deposition by ion implantation or by gas-, plasma- or solid-source diffusion. In some embodiments, the doping of semiconductor layer 256 a-256 c and the formation of source 262 and drain 266 are performed simultaneously. Portions of semiconductor layer 256 a, 256 c disposed over source 262 and drain 266 may have top surfaces substantially free of facets. In an embodiment, portions of source 262, drain 266, and/or gate 259 may be etched away to define recess prior to deposition of semiconductor layer 256 a-256 c, and semiconductor layer 256 a-256 c may then be deposited in the recesses thus formed.

Referring to FIG. 8C, a metal layer 272 is formed over transistor 250. Metal layer 272 is formed by, for example, sputter deposition. Metal layer 272 has a thickness T₂₇₂ of, e.g., 50-200 Å and includes a metal such as cobalt, titanium, tungsten, nickel, or platinum. The metal is selected to react with semiconductor layer 256 a-256 c to form a low-resistance metal-semiconductor alloy when exposed to heat, as described below. The metal is also selected such that the metal-semiconductor alloy remains stable at temperatures typically required to complete transistor 250 fabrication, e.g., 400-700° C.

Referring also to FIG. 8D, subsequent to deposition of metal layer 272, a first rapid thermal anneal is performed, e.g., at 550° C. for 60 seconds. This heating step initiates a reaction between metal layer 272 and semiconductor layers 256 a-256 c, forming a high resistivity phase of a metal-semiconductor alloy, e.g., cobalt silicide (CoSi). Portions of metal layer 272 are removed by a wet etch, such as sulfuric acid and hydrogen peroxide. In an alternative embodiment, the wet etch may be ammonium hydroxide, peroxide, and water. This wet etch removes portions of metal layer 272 disposed over dielectric material, such as over first and second sidewall spacers 252, 254 and isolation regions 268, 270. Portions 274 of metal layer 272 disposed over semiconductor layer 256 a-256 c that have reacted to form the metal-semiconductor alloy remain in place after the anneal and wet etch.

Referring to FIG. 8E, SSOI substrate 100, including transistor 250, is subjected to a second heat treatment. For example, in an embodiment in which metal layer 272 includes cobalt, SSOI substrate 100 undergoes a rapid thermal anneal at 800° C. for 60 seconds in a nitrogen ambient. This heating step initiates a reaction in the metal-semiconductor alloy layer which substantially lowers its resistivity, to form a substantially homogeneous contact layer 276 a-276 c. Contact layer 276 a-276 c includes a metal-semiconductor alloy, e.g., a metal silicide such as a low resistivity phase of cobalt silicide (CoSi₂). Contact layer 276 a-276 c has a thickness T₂₇₆ of, for example, 400 Å. Contact layer 276 a-276 c has a low sheet resistance, e.g., less than about 10 Ω/□, and enables a good quality contact to be made to source 262 and drain 266, as well as to gate 259.

In some embodiments, during formation, contact layer 276 a-276 c may consume substantially all of semiconductor layer 256 a-256 c. A bottommost boundary 278 a of contact layer 276 a, therefore, shares an interface 280 a with strained layer 18 in source 262, and a bottommost boundary 278 c of contact layer 276 c, therefore, shares an interface 280 c with strained layer 18 in drain 266. A bottommost boundary 278 b of contact layer 276 b shares an interface 280 b with gate 259.

In other embodiments, contact layer portions 267 a, 276 c, disposed over source 262 and drain 266, may extend into strained layer 18. Interfaces 280 a, 280 c between contact layer 267 a, 276 c and strained layer 18 are then disposed within source 262 and drain 266, respectively, above bottommost boundaries 282 a, 282 c of strained layer 18. Interfaces 280 a, 280 c have a low contact resistivity, e.g., less than approximately 5×10⁻⁷ Ω-cm². In certain other embodiments, during formation, contact layer 276 a-276 c may not consume all of semiconductor layer 256 a-256 c (see FIG. 8D). A bottommost boundary 278 a of contact layer 267 a, therefore, shares an interface with semiconductor layer 256 a over source 262, and a bottommost boundary 278 c of contact layer 276 c, therefore, shares an interface with semiconductor layer 256 c over drain 266.

Because strained layer 18 includes a strained material, carrier mobilities in strained layer 18 are enhanced, facilitating lower sheet resistances. This strain also results in a reduced energy bandgap, thereby lowering the contact resistivity between the metal-semiconductor alloy and the strained layer.

In alternative embodiments, an SSOI structure may include, instead of a single strained layer, a plurality of semiconductor layers disposed on an insulator layer. For example, referring to FIG. 9, epitaxial wafer 300 includes strained layer 18, relaxed layer 16, graded layer 14, and substrate 12. In addition, a semiconductor layer 310 is disposed over strained layer 18. Strained layer 18 may be tensilely strained and semiconductor layer 310 may be compressively strained. In an alternative embodiment, strained layer 18 may be compressively strained and semiconductor layer 310 may be tensilely strained. Strain may be induced by lattice mismatch with respect to an adjacent layer, as described above, or mechanically. For example, strain may be induced by the deposition of overlayers, such as Si₃N₄. In another embodiment, semiconductor layer 310 is relaxed. Semiconductor layer 310 includes a semiconductor material, such as at least one of a group II, a group III, a group IV, a group V, and a group VI element. Epitaxial wafer 300 is processed in a manner analogous to the processing of epitaxial wafer 8, as described with reference to FIGS. 1-7.

Referring also to FIG. 10, processing of epitaxial wafer 300 results in the formation of SSOI substrate 350, having strained layer 18 disposed over semiconductor layer 310. Semiconductor layer 310 is bonded to dielectric layer 52, disposed over substrate 54. As noted above with reference to FIG. 9, strained layer 18 may be tensilely strained and semiconductor layer 310 may be compressively strained. Alternatively, strained layer 18 may be compressively strained and semiconductor layer 310 may be tensilely strained. In some embodiments, semiconductor layer 310 may be relaxed.

Referring to FIG. 11, in some embodiments, a thin strained layer 84 may be grown between strained layer 18 and relaxed layer 16 to act as an etch stop during etching, such as wet etching. In an embodiment in which strained layer 18 includes Si and relaxed layer 16 includes Si_(1−y)Ge_(y), thin strained layer 84 may include Si_(1−x)Ge_(x), with a higher Ge content (x) than the Ge content (y) of relaxed layer 16, and hence be compressively strained. For example, if the composition of the relaxed layer 16 is 20% Ge (Si_(08.0)Ge_(0.20)), thin strained layer 84 may contain 40% Ge (Si_(0.60)Ge_(0.40)) to provide a more robust etch stop. In other embodiments, a second strained layer, such as thin strained layer 84 with higher Ge content than relaxed layer 16, may act as a preferential cleave plane in the hydrogen exfoliation/cleaving procedure described above.

In an alternative embodiment, thin strained layer 84 may contain Si_(1−x)Ge_(x) with lower Ge content than relaxed layer 16. In this embodiment, thin strained layer 84 may act as a diffusion barrier during the wet oxidation process. For example, if the composition of relaxed layer 16 is 20% Ge (Si_(0.80)Ge_(0.20)), thin strained layer 84 may contain 10% Ge (Si_(0.90)Ge_(0.10)) to provide a barrier to Ge diffusion from the higher Ge content relaxed layer 16 during the oxidation process. In another embodiment, thin strained layer 84 may be replaced with a thin graded Si_(1−z)Ge_(z) layer in which the Ge composition (z) of the graded layer is decreased from relaxed layer 16 to the strained layer 18.

Referring again to FIG. 7, in some embodiments, a small amount, e.g., approximately 20-100 Å, of strained layer 18 may be removed at an interface 105 between strained layer 18 and relaxed layer portion 80. This may be achieved by overetching after relaxed layer portion 80 is removed. Alternatively, this removal of strained layer 18 may be performed by a standard microelectronics clean step such as an RCA SC1, i.e., hydrogen peroxide, ammonium hydroxide, and water (H₂O₂+NH₄OH+H₂O), which at, e.g., 80° C. may remove silicon. This silicon removal may remove any misfit dislocations that formed at the original strained layer 18/relaxed layer 80 interface 105 if strained layer 18 was grown above the critical thickness. The critical thickness may be defined as the thickness of strained layer 18 beyond which it becomes energetically favorable for the strain in the layer to partially relax via the introduction of misfit dislocations at interface 105 between strained layer 18 and relaxed layer 16. Thus, the method illustrated in FIGS. 1-7 provides a technique for obtaining strained layers above a critical thickness without misfit dislocations that may compromise the performance of deeply scaled MOSFET devices.

Referring to FIG. 12, in some embodiments, handle wafer 50 may have a structure other than a dielectric layer 52 disposed over a semiconductor substrate 54. For example, a bulk relaxed substrate 400 may comprise a bulk material 410 such as a semiconductor material, e.g., bulk silicon. Alternatively, bulk material 410 may be a bulk dielectric material, such as Al₂O₃ (e.g., alumina or sapphire) or SiO₂ (e.g., quartz). Epitaxial wafer 8 may then be bonded to handle wafer 400 (as described above with reference to FIGS. 1-6), with strained layer 18 being bonded to the bulk material 410 comprising handle wafer 400. In embodiments in which bulk material 410 is a semiconductor, to facilitate this semiconductor-semiconductor bond, a hydrophobic clean may be performed, such as an HF dip after an RCA SC1 clean.

Referring to FIG. 13, after bonding and further processing (as described above), a strained-semiconductor-on-semiconductor (SSOS) substrate 420 is formed, having strained layer 18 disposed in contact with relaxed substrate 400. The strain of strained layer 18 is not induced by underlying relaxed substrate 400, and is independent of any lattice mismatch between strained layer 18 and relaxed substrate 400. In an embodiment, strained layer 18 and relaxed substrate 400 include the same semiconductor material, e.g., silicon. Relaxed substrate 400 may have a lattice constant equal to a lattice constant of strained layer 18 in the absence of strain. Strained layer 18 may have a strain greater than approximately 1×10⁻³. Strained layer 18 may have been formed by epitaxy, and may have a thickness T₅ of between approximately 20 Å-1000 Å, with a thickness uniformity of better than approximately ±10%. In an embodiment, strained layer 18 may have a thickness uniformity of better than approximately ±5%. Surface 92 of strained layer 18 may have a surface roughness of less than 20 Å.

Referring to FIG. 14, in an embodiment, after fabrication of the SSOI structure 100 including semiconductor substrate 54 and dielectric layer 52, it may be favorable to selectively relax the strain in at least a portion of strained layer 18. This could be accomplished by introducing a plurality of ions by, e.g., ion implantation after a photolithography step in which at least a portion of the structure is masked by, for example, a photoresist feature 500. Ion implantation parameters may be, for example, an implant of Si ions at a dose of 1×10¹⁵-1×¹⁷ ions/cm², at an energy of 5-75 keV. After ion implantation, a relaxed portion 502 of strained layer 18 is relaxed, while a strained portion 504 of strained layer 18 remains strained.

Devices

In addition to the transistors described above with reference to FIGS. 8A-8E, various other transistors may be formed on SSOI substrate 100 fabricated by the methods described above All of these transistors may also be formed on SSOI substrate 100 fabricated with the use of a porous semiconductor substrate, as described below with reference to FIGS. 40A-41D.

finFET

A finFET (or any variant of the basic finFET structure such as the wrap-around gate FET, tri-gate FET, or omega FET) may be fabricated on SSOI substrate 100 as described below. The finFET and related devices include two gates located on either side of a FET channel region. Unlike in a traditional planar FET, this channel region is raised above the wafer surface: the channel (or portions of the channel) falls in a plane perpendicular to the wafer surface. There may in addition be gates above and/or below the channel region, such as in the wrap-around gate FET.

Referring to FIG. 15, SSOI substrate 100 includes strained layer 18 and dielectric layer 52 disposed over substrate 54. In an embodiment, strained layer 18 includes Si and has thickness T₆ of, e.g., 200-1000 Å. Dielectric layer 52 may be formed from SiO₂, with thickness T₇ selected from the range of, e.g., 500-3000 Å. Substrate 54 may be formed from, e.g., Si.

Referring to FIGS. 16A and 16B, strained layer 18 is patterned to define a plurality of fins 600. Fins 600 are defined by the formation of a photolithographic mask (not shown) over strained layer 18, followed by anisotropic reactive ion etching (RIE) of strained layer 18. Fins 600 have a width W₁ of, e.g., 50-300 Å. The photomask/RIE steps also define source mesa region 602 and drain mesa region 604. Fins 600, source mesa region 602, and source mesa region 604 include portions of strained layer 18 not removed by RIE. The photolithographic mask is removed after the RIE of strained layer 18.

Referring to FIG. 17, a gate insulator layer 610 is formed over SSOI substrate 100. Gate insulator layer 610 is conformally formed over fins 600, as well as over source and drain mesa regions 602, 604. Gate insulator layer 610 may include, e.g., thermally grown SiO₂, or a high-k dielectric like HfO₂ or HfSiON, and have a thickness T₈ of, e.g., 10-100 Å. In some embodiments, gate insulator layer 610 is grown, and is therefore formed only over exposed silicon surfaces, i.e., over fins 600 and source and drain mesa regions 602, 604. In other embodiments, gate insulator layer 610 is deposited, and is therefore formed over an entire top surface of SSOI substrate 100.

Referring to FIGS. 18A and 18B, a gate electrode material 620 is conformally formed over gate insulator layer 610, including over fins 600. Gate electrode material 620 may be, e.g., polycrystalline silicon (“polysilicon”), deposited by CVD, such as by UHVCVD, APCVD, LPCVD, or PECVD, having a thickness T₆₂ selected from the range of, e.g., 100-2000 Å. A photolithographic mask (not shown) is formed over gate electrode material 620. Portions of gate electrode material 620 are selectively removed by, e.g., RIE to define a gate 622 crossing over fins 600, and terminating in a gate contact area 624. Portions of gate insulator layer 610 are exposed (or even removed) by the RIE of gate electrode material 620.

Referring to FIGS. 19A and 19B, a plurality of dopants are introduced into source and drain mesa regions 602, 604 to define source 630 and drain 632. To form an n-type finFET, dopants such as arsenic or phosphorus may be implanted into mesa regions 602, 604. Possible implantation parameters may be, for example, arsenic with a dose of 2×10¹⁵ atoms/cm² implanted at an energy of 10-50 kilo-electron volts (keV). To form a p-type finFET, dopants such as boron may be implanted into mesa regions 602, 604. Possible implantation parameters may be, for example, boron, with a dose of 2×10¹⁵ atoms/cm² at an energy of 3-15 keV. For the formation of a CMOS device, NMOS regions may be protected by a mask during the implantation of p-type dopants into PMOS regions. Similarly, PMOS regions may be protected by a mask during the implantation of n-type dopants into NMOS regions. A suitable mask for both types of implantation may be, e.g., photoresist.

During the introduction of dopants into source and drain mesa regions 602, 604, a plurality of gate dopants 634 are also introduced into gate 622 and gate contact area 624. Gate dopants 634 serve to increase a conductivity of gate electrode material 620. Gate dopants 630 may be, for example, implanted arsenic or phosphorous ions for an n-type finFET.

Dopants for both n-type and p-type finFETs may be implanted at an angle of 20-50°, with zero degrees being normal to SSOI substrate 100. Implanting at an angle may be desired in order to implant ions into a side of exposed fins 600 and also into a side of the vertical surfaces of gate electrode material 620.

Referring to FIGS. 20A and 20B, a blanket layer of spacer insulator material is formed over SSOI substrate 100, including over gate 622, gate contact 624, source 630, and drain 632. Spacer insulator material may be, for example, SiO₂ or Si₃N₄ deposited by CVD and have a thickness T₉ of, for example, 100-1000 Å. Subsequently, portions of spacer insulator material are removed by an anisotropic RIE to define a plurality of sidewall spacers 642 proximate vertical surfaces, such as fins 600, gate 622, and gate contact area 624. Horizontal surfaces, such as top surfaces of fins 600, are substantially free of the spacer insulator material.

After the RIE definition of sidewall spacers 642, the portions of gate insulator layer 610 exposed by the RIE of gate electrode material 620 may be removed from top surfaces of source 630, and drain 632 by, e.g., a dip in hydrofluoric acid (HF), such as for 5-30 seconds in a solution containing, e.g., 0.5-5% HF. Alternately, this removal may be via RIE, with an etchant species such as, e.g., CHF₃.

Referring to FIGS. 21A and 21B, a self-aligned silicide (“salicide”) is formed over SSOI substrate 100 to provide low resistance contacts as follows. A conductive layer is formed over SSOI substrate 100. For example, a metal such as cobalt or nickel is deposited by, e.g., CVD or sputtering, with the conductive layer having a thickness of, e.g., 50-200 Å. An anneal is performed to react the conductive layer with the underlying semiconductor, e.g., exposed portions of gate 622 and gate contact area 624, to form salicide 650 including, e.g., cobalt silicide or nickel silicide. Anneal parameters may be, for example, 400-800° C. for 10-120 seconds. Unreacted portions of the conductive layer disposed directly over insulator material, such as exposed portions of dielectric layer 52 and sidewall spacers 642, are removed by a chemical strip. A suitable chemical strip may be a solution including H₂SO₄:H₂O₂ at a ratio of 3:1. A second anneal may be performed to further lower resistivity of salicide 650. The second anneal parameters may be, for example, 600-900° C. for 10-120 seconds. A finFET 655 includes fins 600, gate insulator 610, source 630, drain 632, and gate 622. A finFET 655 having three fins 600 is illustrated in FIG. 21B. The three fins 600 share a common source 630 and a common drain 632. A single transistor may have multiple fins to increase current drive in comparison to a transistor with a single fin.

In an alternative embodiment, gate dielectric material may be removed from the top surfaces of the source and drain mesa regions immediately after the RIE of the gate electrode. In some embodiments, raised source and drain regions may be formed, as described above with reference to FIGS. 8B-8D.

Double Gate MOSFETs

Referring to FIG. 22 as well as to FIG. 1A, epitaxial wafer 8 has layers 10 disposed over substrate 12. Substrate 12 may be formed of a semiconductor, such as Si, Ge, or SiGe. The plurality of layers 10 includes graded buffer layer 14, formed of Si_(1−y)Ge_(y), with a maximum Ge content of, e.g., 10-80% (i.e., y=0.1-0.8). Relaxed layer 16 is disposed over graded buffer layer 14. Relaxed layer 16 may be formed of uniform Si_(1−x)Ge_(x) having a Ge content of, for example, 10-80% (i.e., x=0.1-0.8). Strained semiconductor layer 18 is disposed over relaxed layer 16. Strained layer 18 comprises at least one of a group II, a group III, a group IV, a group V, and a group VI element. Strained layer 18 may include, for example, Si and may be tensilely strained.

A first gate insulator layer 700 is formed over strained layer 18. First gate insulator layer 700 may include SiO₂ or a high-k dielectric like HfO₂ or HfSiON, and may be grown or deposited. First gate insulator layer 700 may have a thickness T₁₁ of, e.g., 10-100 Å. A first gate electrode layer 702 is formed over first gate insulator layer 700. First gate electrode layer 702 may include a conductive material, for example, doped polycrystalline silicon or tungsten, and may have a thickness T₁₂ of, for example, 500-2000 Å.

Referring to FIG. 23, ions 704 are introduced to define cleave plane 20 in relaxed layer 16, in the manner described above with reference to FIG. 2A.

Referring to FIG. 24, epitaxial wafer 8 is bonded to handle wafer 50, in the manner described above with reference to FIG. 3. Handle wafer 50 includes dielectric layer 52 disposed over semiconductor substrate 54.

Referring to FIG. 25 as well as to FIG. 24, the bond between epitaxial wafer 8 and handle wafer 50 may be strengthened by an anneal at a relatively low temperature such as, e.g., 200-300° C. Epitaxial wafer 8 is separated from handle wafer 50 by inducing a split along cleave plane 20 with an anneal at, e.g., 300-700° C. After cleaving, a SSOI substrate 710 includes strained layer 18 disposed over first gate insulator 700, first gate electrode layer 702, insulator 52, and substrate 54. Residual portion 80 of relaxed layer 16 is disposed over strained layer 18. Relaxed layer portion 80 is selectively removed by, e.g., thermal oxidation and HF strip in the manner discussed above with reference to FIGS. 4 and 5.

Referring to FIG. 26, a second gate insulator layer 720 is formed over strained layer 18. Second gate insulator layer 720 may include SiO₂ or a high-k dielectric like HfO₂ or HfSiON, and may be grown or deposited. First gate insulator layer 720 may have a thickness T₁₃ of, e.g., 10-100 Å. A second gate electrode layer 722 is formed over second gate insulator layer 720. Second gate electrode layer 722 may include a conductive material such as, for example, doped polycrystalline silicon, and may have a thickness T₁₄ of, for example, 500-2000 Å.

Referring to FIG. 27 as well as to FIG. 26, second gate electrode layer 722 is patterned by photolithography and RIE to define a second gate electrode 730. A source 732 and a drain 734 are formed in strained layer 18 by, e.g., implanting dopants, such as n-type or p-type dopants, into strained layer 18. A spacer dielectric layer is deposited and etched back to define dielectric sidewall spacers 736 proximate second gate electrode 730.

Referring to FIG. 28, a conductive spacer layer 740 is deposited over strained layer 18, second gate electrode 730, and dielectric sidewall spacers 736. Conductive spacer layer 740 includes a conductive material, such as doped polycrystalline silicon or a metal. Conductive spacer layer 740 has a thickness T₁₅ of, e.g., 500-2000 Å.

Referring to FIG. 29 as well as to FIG. 28, conductive spacer layer 740 is anisotropically etched to form conductive sidewall spacers 742, proximate dielectric sidewall spacers 736.

Referring to FIG. 30 as well as to FIG. 29, an RIE is performed to remove portions of strained layer 18, first gate insulator layer 700, and first gate electrode layer 702 not disposed directly below second gate electrode 730, dielectric sidewall spacers 736, and conductive sidewall spacers 742. After this RIE, a vertical structure 744 includes strained layer 18, first gate insulator layer 700, and first gate electrode layer 702 regions disposed under second gate electrode 730 and sidewall spacers 736, 742. Vertical structure 744 has a width W₂ of, e.g., 1000-5000 Å

Referring to FIG. 31, an isotropic etch is performed to laterally shrink first gate electrode layer 702 region disposed under second gate electrode 730, thus defining first gate electrode 750. This isotropic etch may be a wet etch, such as hydrogen peroxide (in an embodiment in which first gate electrode layer 702 includes tungsten) or an isotropic dry etch. The width of first gate electrode layer 702 may be reduced such that both the first gate electrode 750 and the second gate electrode 730 have approximately the same width W₃ that is less than W₂, e.g., 100-2000 Å.

Referring to FIG. 32, a thick insulator layer 760 is deposited over insulator layer 52 and vertical structure 744, i.e., over second gate electrode 730 and conductive sidewall spacers 742, as well as proximate strained layer 18, first gate insulator layer 700, and first gate electrode 750. Thick insulator layer 760 has an initial thickness T₁₆ over insulator 52 of, e.g., 5000 Å. Thick insulator layer 760 is then planarized by, e.g., CMP.

Referring to FIGS. 33-35, contact holes 770 are formed through thick insulator layer 760 to conductive sidewall spacers 742 and second gate electrode 730. Contact holes 770 may be defined by the use of photolithography and RIE. Contact holes 770 are filled with a conductive material such as, e.g., a metal such as titanium or tungsten. The conductive material is patterned by photolithography and etch to define contacts 780 to source 732, drain 734, first gate electrode 750 at a first gate electrode 793, and second gate electrode 730 at a second gate electrode 795. Double gate transistor 790 includes first gate electrode 750, second gate electrode 730, first gate insulator layer 700, second gate insulator layer 722, source 732, and drain 734.

Heterojunction Bipolar Transistor

Referring to FIG. 36 as well as to FIG. 6, a heterojunction bipolar transistor (HBT) may be formed on SSOI substrate 100, including strained layer 18, dielectric layer 52, and substrate 54. A collector 810 for the HBT is formed in a portion of strained layer 18 by the introduction of dopants into the strained layer 18 portion. Collector 810 includes a low-doped region 811 and a high-doped region 812. Low-doped region 811 is doped at a relatively low level, for example at 5×10¹⁶-1×10¹⁸ atoms/cm³, and has a thickness T₂₀ of, for example, 100-1000 Å. High-doped region 812 is doped to a level not less than the doping level of low-doped region 811, preferably to a relatively high level of, e.g., 1×10¹⁹-1×10²¹ atoms/cm³. Low-doped region 811 and high-doped region 812 are doped with the same type of dopants, and both may be doped either n-type or p-type. In an embodiment, both regions are doped n-type. Collector 810 may be electrically isolated from other devices formed on the substrate through the use of, for example, trench isolation (not shown).

A total thickness T₂₁ of collector 810 may be increased to improve performance by subsequent additional deposition of a material that is lattice matched to the original strained layer 18 portion. The additional material may be, for example, SiGe lattice-matched to strained layer 18.

Referring to FIG. 37, a masking layer is formed over collector 810. The masking layer may include a dielectric material, such as, e.g., SiO₂ or Si₃N₄. Photoresist is disposed over the masking layer and patterned to expose an area of the masking layer. This area is removed by, e.g., wet etching or RIE, to define a mask 910 disposed over strained layer 18. Mask 910 exposes a region 920 of collector 810.

Referring to FIG. 38, a base 1010 is formed over region 920 of collector 810. Base 1010 may be formed selectively by, e.g., selective deposition of a semiconductor material only over region 920 defined by mask 910. The selective deposition can be done by CVD methods, such as by APCVD, LPCVD, UHVCVD, or by MBE. In an embodiment, base 1010 may be deposited non-selectively. The non-selectively grown material will thus also form on a top surface 1012 of mask 910, and may be removed by further photolithography and etch steps. Base 1010 has a thickness T₂₂ of, e.g., of 50-1000 Å. In an embodiment, T₂₂ may be, for example 30-500 Å. Base 1010 includes a semiconductor material like Si or SiGe. In some embodiments, base 1010 is relaxed or compressively strained. The in-plane lattice constant of collector 810 (strained layer 18) was defined by relaxed layer 16 (see FIG. 1A). Therefore, in order that base 1010 be relaxed, the Ge content of base 1010 should be equal to the Ge content of relaxed layer 16 (see FIG. 1A). Similarly, in order that base 1010 be compressively strained, the Ge content of base 1010 should be greater than the Ge content of relaxed layer 16. This difference in Ge content also provides a base 1010 with a bandgap no larger than that of collector 810, which can be advantageous to device operation. In other embodiments, base 1010 is tensilely strained. In order that base 1010 be tensilely strained, the Ge content of base 1010 should be less than the Ge content of relaxed layer 16 (see FIG. 1A). Alternatively, base 1010 may be formed from the same material as collector 810, for example strained Si. Base 1010 is doped the opposite doping type as the collector, i.e., base 1010 is p-type doped for an n-type doped collector. Base 1010 may be doped during the deposition process, but may also be doped after deposition by ion implantation. Base 1010 may be doped to a level of 1×10¹⁸-1×10¹⁹ atoms/cm³.

In some embodiments, the base doping may be significantly higher, e.g., ≧10²⁰ atoms/cm³. In such embodiments, the outdiffusion of dopants from base 1010 may be deleterious to device performance, and therefore the p-type doping of base 1010 may be reduced within base 1010 in regions adjacent to an emitter 1110/base 1010 interface (see FIG. 39) and a base 1010/collector 810 interface 1014. These regions with reduced doping may have thicknesses of, e.g., 10 Å-30 Å.

In an embodiment, base 1010 contains an element with a concentration of 1×10¹⁸-1×10²⁰ atoms/cm³ that suppresses the diffusion of dopants out of base 1010 during subsequent high temperature processing steps. A suitable element for diffusion suppression may be, for example, carbon. In another embodiment, base 1010 may be formed of SiGe, with the Ge content of base 1010 being not uniform across the thickness of base 1010. In this case, the Ge content of base 1010 may be graded in concentration, with higher Ge content at base-collector interface 1014 and lower Ge content at a base upper surface 1016. In other embodiments, the Ge content of base 1010 can have a trapezoidal or triangular profile.

Referring to FIG. 39, an emitter 1110 is formed on base 1010. Emitter 1110 may be formed by the deposition of a semiconductor layer over base 1010 and mask 910. The semiconductor layer may be subsequently patterned by photolithographic and etch steps to define emitter 1110. Emitter 1110 may include a semiconductor material such as Si or SiGe, and may have a Ge content lower than the Ge content of base 1010. In an embodiment, emitter 1110 has a Ge content equal to that of relaxed layer 16 (see FIG. 1A) that originally defined the in-plane lattice constant of strained layer 18 (and hence collector 810). In another embodiment, the Ge content of emitter 1110 may be lower than that of relaxed layer 16, and, therefore, emitter 1110 is tensilely strained. In another embodiment, emitter 1110 may include the same material as strained layer 18/collector 810, such as, for example, strained Si.

Emitter 1110 has two regions: an upper emitter region 1111 and a lower emitter region 1112. Lower emitter region 1112 has a thickness T₂₃ of 10-2000 Å and is doped with a same doping type as collector 810 (and hence the opposite doping type of base 1010). For example, lower emitter region 1112 and collector 810 may be doped n-type and base 1010 may be doped p-type. Lower emitter region 1112 may be doped at a concentration of 1×10¹⁷-5×10¹⁸ atoms/cm³, for example 1×10¹⁸ atoms/cm³. Upper emitter region 1111 has a thickness T₂₄ of, for example, 100-4000 Å and is doped the same doping type as lower emitter region 1112. Upper emitter region 1111 may be doped at a concentration of 1×10¹⁹-1×10²¹ atoms/cm³, for example 1×10²⁰-5×10²⁰ atoms/cm³. An HBT 1200 includes collector 810, base 1010, and emitter 1110.

After formation of emitter 1110, metal contacts (not shown) may be made to each of collector 810, base 1010, and emitter 1110. Mask 910 may be removed or further patterned during the formation of metal contacts. HBT 1200 may be a standalone device or may be interconnected to other devices fabricated on SSOI substrate 100, such as, for example, transistor 200 (see FIG. 8A), finFET 655 (see FIGS. 21A and 21B), or double-gate transistor 790 (see FIG. 33).

In an embodiment, HBT 1200 may be formed on SSOS substrate 420 (see FIG. 13) by the steps described above with reference to FIGS. 36-39. In another embodiment, HBT 1200 may be formed on relaxed portion 504 of strained layer 18 (see FIG. 14) by the steps described above with reference to FIGS. 36-39. In this embodiment, collector 810 is formed in relaxed portion 504.

In another embodiment, HBT 1200 may be formed on a region of SSOI substrate 100 (see FIG. 6) in which portions of strained layer 18 and dielectric layer 52 have been removed by the steps described with reference to FIGS. 36-39. In this embodiment, collector 810 is formed in substrate 54 and may be increased in thickness by deposition of another semiconductor layer as described above. This configuration enables the interconnection of HBT 1200 formed directly on semiconductor substrate 54 with devices formed on other portions of SSOI substrate 100, for example transistor 200 of FIG. 8A.

Formation of SSOI Substrate by use of a Porous Semiconductor Substrate

Referring to FIGS. 40A-40E, SSOI structure 100 (see FIG. 6) may be formed by the use of a porous semiconductor substrate. Referring to FIG. 40A, substrate 12 may be formed of a semiconductor, such as Si, Ge, or SiGe. A plurality of pores 1514, i.e., microvoids, are formed to define a porous layer 1516 in a portion of substrate 12. Pores 1514 may have a median diameter of 5-10 nm and a pitch of 10-50 nm. Porous layer 1516 may have a porosity of 10-50% and may extend a depth of d₁₅ into substrate 12 of approximately 1-5 μm.

Referring to FIG. 40B, pores 1514 may be formed by, for example, submerging substrate 12 into a vessel 1517 containing an electrolyte 1518, such as hydrofluoric acid (HF), possibly mixed with ethanol, with a cathode 1520 and an anode 1522 disposed in the electrolyte 1518. A back surface chucking holder 1519 a with a vacuum pad 1519 b may hold substrate 12 while it is submerged in vessel 1517. A current may be generated between cathode 1520 and anode 1522, through substrate 12, resulting in the electrochemical etching of substrate 12, thereby forming pores 1514 at a top surface 1524 of substrate 12. In an embodiment, prior to the formation of pores 1514, substrate 12 may be planarized, e.g., by CMP.

Referring to FIG. 40C, after the formation of pores 1514, a plurality of layers 10 may be formed over porous top surface 1524 of substrate 12, as described with reference to FIG. 1A. Layers 10 may include, for example, graded buffer layer 14, relaxed layer 16, and strained layer 18. Pores 1514 define cleave plane 20 in porous layer 1516 of substrate 12.

Referring to FIG. 40D, substrate 12 with layers 10 is bonded to handle wafer 50, including semiconductor substrate 54 and dielectric layer 52, as described with reference to FIG. 3. Prior to bonding, a dielectric layer may be formed on a top surface of layers 10 to facilitate the bonding process and to serve as an insulator layer in the final substrate structure.

Referring to FIG. 40E as well as to FIG. 40D, a split is induced at cleave plane 20 by, for example, cleaving porous layer 1516 by a water or an airjet. The split results in the formation of two separate wafers 1570, 1572. One of these wafers (1572) has graded layer 14 and relaxed layer 16 (see FIG. 40 c) disposed over strained layer 18, with a first portion 1580 of substrate 12 disposed over graded layer 14. First portion 1580 of substrate 12 may be just trace amounts of material surrounding pores 1514. Strained layer 18 is in contact with dielectric layer 52 on semiconductor substrate 54. The other of these wafers (1570) includes a second portion 1582 of substrate 12, including the bulk of substrate 12 with perhaps trace amounts of material surrounding pores 1514.

Referring to FIG. 6 as well as to FIG. 40E, first portion 1580 of substrate 12 is removed from graded layer 14 by a wet chemical cleaning process utilizing, for example a mixture of hydrogen peroxide (H₂O₂) and HF. Graded layer 14 and relaxed layer 16 are removed in any one of the methods described for the removal of relaxed layer portion 80 with reference to FIGS. 4 and 5. Removal of graded and relaxed layers 14, 16 results in the formation of SSOI substrate 100.

Referring to FIG. 41A, SSOI substrate 100 (see FIG. 6) may also be formed by the use of porous intermediate layers. For example, plurality of layers 10 may be formed over substrate 12, layers 10 including graded layer 14, relaxed layer 16, and strained layer 18 (see FIG. 1A). Prior to the formation of strained layer 18, a plurality of pores 1614 may be formed in a top portion of relaxed layer 16, thereby defining a porous layer 1616 in a top portion 1617 of relaxed layer 16. Pores 1614 may be formed by the methods described above with reference to the formation of pores 1514 in FIG. 40B. Porous layer 1616 may have a thickness T₁₆ of, e.g., 1-5 μm. Strained layer 18 may then be formed directly over porous layer 1616. Pores 1614 define cleave plane 20 in porous layer 1616.

Referring to FIG. 41B, in an alternative embodiment, after the formation of porous layer 1616 in a portion of relaxed layer 16, a second relaxed layer 1620 may be formed over relaxed layer 16 including porous layer 1616. Second relaxed layer 1620 may include the same material from which relaxed layer 16 is formed, e.g., uniform Si_(1−x)Ge_(x) having a Ge content of, for example, 10-80% (i.e., x=0.1-0.8) and having a thickness T₁₇ of, e.g., 5-100 nm. In some embodiments, Si_(1−x)Ge_(x) may include Si_(0.70)Ge_(0.30) and T₁₇ may be approximately 50 nm. Second relaxed layer 1620 may be fully relaxed, as determined by triple axis X-ray diffraction, and may have a threading dislocation density of <1×10⁶/cm², as determined by etch pit density (EPD) analysis. Strained layer 18 may be formed over second relaxed layer 1620. Pores 1614 define cleave plane 20 in porous layer 1616.

Referring to FIG. 41C, substrate 12 with layers 10 is bonded to handle wafer 50, including semiconductor substrate 54 and dielectric layer 52, as described with reference to FIG. 3.

Referring to FIG. 41D as well as to FIG. 41C, a split is induced at cleave plane 20 by, for example, cleaving porous layer 1616 by a water or an airjet. The split results in the formation of two separate wafers 1670, 1672. One of these wafers (1670) has top portion 1617 of relaxed layer 16 (see FIG. 41A) disposed over strained layer 18. Strained layer 18 is in contact with dielectric layer 52 on semiconductor substrate 54. The other of these wafers (1672) includes the substrate 12, graded layer 14, and a bottom portion 1674 of relaxed layer 16.

Referring to FIG. 6 as well as to FIG. 41D, top portion 1617 of relaxed layer 16 is removed in any one of the methods described for the removal of relaxed layer portion 80 with reference to FIGS. 4 and 5. Removal of top portion 1617 of relaxed layer 16 results in the formation of SSOI substrate 100.

The bonding of strained silicon layer 18 to dielectric layer 52 has been experimentally demonstrated. For example, strained layer 18 having a thickness of 54 nanometers (nm) along with ˜350 nm of Si_(0.70)Ge_(0.30) have been transferred by hydrogen exfoliation to Si handle wafer 50 having dielectric layer 52 formed from thermal SiO₂ with a thickness of approximately 100 nm. The implant conditions were a dose of 4×10¹⁶ ions/cm³ of H₂ ⁺ at 75 keV. The anneal procedure was 1 hour at 550° C. to split the SiGe layer, followed by a 1 hour, 800° C. strengthening anneal. The integrity of strained Si layer 18 and good bonding to dielectric layer 52 after layer transfer and anneal were confirmed with cross-sectional transmission electron microscopy (XTEM). An SSOI structure 100 was characterized by XTEM and analyzed via Raman spectroscopy to determine the strain level of the transferred strained Si layer 18. An XTEM image of the transferred intermediate SiGe/strained Si/SiO₂ structure showed transfer of the 54 nm strained Si layer 18 and ˜350 nm of the Si_(0.70)Ge_(0.30) relaxed layer 16. Strained Si layer 18 had a good integrity and bonded well to SiO₂ 54 layer after the annealing process.

XTEM micrographs confirmed the complete removal of relaxed SiGe layer 16 after oxidation and HF etching. The final structure includes strained Si layer 18 having a thickness of 49 nm on dielectric layer 52 including SiO₂ and having a thickness of 100 nm.

Raman spectroscopy data enabled a comparison of the bonded and cleaved structure before and after SiGe layer 16 removal. Based on peak positions the compostion of the relaxed SiGe layer and strain in the Si layer may be calculated. See, for example, J. C. Tsang, et al., J. Appl. Phys. 75 (12) p. 8098 (1994), incorporated herein by reference. The fabricated SSOI structure 100 had a clear strained Si peak visible at ˜511/cm. Thus, the SSOI structure 100 maintained greater than 1% tensile strain in the absence of the relaxed SiGe layer 16. In addition, the absence of Ge—Ge, Si—Ge, and Si—Si relaxed SiGe Raman peaks in the SSOI structure confirmed the complete removal of SiGe layer 16.

In addition, the thermal stability of the strained Si layer was evaluated after a 3 minute 1000° C. rapid thermal anneal (RTA) to simulate an aggregate thermal budget of a CMOS process. A Raman spectroscopy comparision was made of SSOI structure 100 as processed and after the RTA step. A scan of the as-bonded and cleaved sample prior to SiGe layer removal was used for comparision. Throughout the SSOI structure 100 fabrication processs and subsequent anneal, the strained Si peak was visible and the peak position did not shift. Thus, the strain in SSOI structure 100 was stable and was not diminished by thermal processing. Furthermore, bubbles or flaking of the strained Si surface 18 were not observed by Nomarski optical microscopy after the RTA, indicating good mechanical stability of SSOI structure 100.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments are therefore to be considered in all respects illustrative rather than limiting on the invention described herein. Scope of the invention is thus indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein. 

1-12. (canceled)
 13. A structure comprising: a dielectric layer disposed over a substrate; and a transistor formed over the dielectric layer, the transistor including: a first gate electrode disposed over the dielectric layer; a strained semiconductor layer disposed over the first gate electrode; and a second gate electrode disposed over the strained semiconductor layer and disposed over the first gate electrode.
 14. The structure of claim 13, wherein the strained semiconductor layer comprises at least one of a group II, a group III, a group IV, a group V, and a group VI element.
 15. The structure of claim 13, wherein the strained semiconductor layer is tensilely strained.
 16. The structure of claim 15, wherein the strained semiconductor layer comprises tensilely strained silicon.
 17. The structure of claim 13, wherein the strained semiconductor layer is compressively strained.
 18. The structure of claim 17, wherein the strained semiconductor layer comprises compressively strained germanium.
 19. The structure of claim 13, wherein the strained semiconductor layer has a strain level greater than 10⁻³.
 20. The structure of claim 13, further comprising: a first gate insulator layer disposed between the first gate electrode and the strained semiconductor layer.
 21. The structure of claim 20, further comprising: a second gate insulator layer disposed between the strained semiconductor layer and the second gate electrode.
 22. The structure of claim 13, wherein the strained semiconductor layer comprises a source and a drain.
 23. (canceled)
 24. The structure of claim 13, further comprising: a sidewall spacer disposed proximate the second gate electrode.
 25. The structure of claim 24, wherein the sidewall spacer comprises a dielectric material.
 26. The structure of claim 24, wherein the sidewall spacer comprises a conductive material. 27-62. (canceled)
 63. The structure of claim 13, wherein the first gate electrode is in contact with the dielectric layer.
 64. The structure of claim 13, wherein the first gate electrode comprises a conductive material.
 65. The structure of claim 64, wherein the conductive material is selected from the group consisting of doped polycrystalline silicon and a metal.
 66. The structure of claim 13, wherein the second gate electrode comprises a conductive material.
 67. The structure of claim 66, wherein the conductive material is selected from the group consisting of polycrystalline silicon and a metal.
 68. The structure of claim 21, wherein the first gate insulator layer comprises a material having a higher dielectric constant higher than a dielectric constant of silicon dioxide.
 69. The structure of claim 21, wherein the first gate insulator layer is in contact with the strained semiconductor layer.
 70. The structure of claim 21, wherein the second gate insulator layer comprises a material having a higher dielectric constant higher than a dielectric constant of silicon dioxide.
 71. The structure of claim 21, wherein the second gate insulator layer is in contact with the strained semiconductor layer. 